JP2005087375A - Magnetic resonance imaging apparatus and magnetic resonance image generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus and a magnetic resonance image generation method capable of mitigating the limitation of space selectivity and frequency selectivity in the case of space-selectively and frequency-selectively generating magnetic resonance signals. <P>SOLUTION: At the time of impressing a first pulse sequence PS1 for suppressing the frequency components of fat in a prescribed slice of a part to be examined of a subject by an RF coil part and a gradient coil part, gradient magnetic field pulses 51a for which a positive pulse PLP and negative pulse PLN of the same area ar1 are alternately and successively connected and the ratio of the size HT1 of the positive pulse PLP and the size HT2 of the negative pulse PLN is expressed by HT1:HT2=1:2 is impressed by the gradient coil part, and the respective pulses of RF waves 50a for space-selective frequency selection are impressed by the RF coil part together with the positive pulse PLP. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance image generation method for performing imaging by generating a magnetic resonance signal in which a specific frequency component is suppressed or excited from a specific region of a test site of a subject.

  Magnetic Resonance Imaging (MRI) applies a gradient magnetic field and RF (Radio Frequency) waves to a subject in a static magnetic field, and images based on magnetic resonance signals emitted as echoes from protons at the subject site. Is a technology to generate

As a type of magnetic resonance imaging, a method of collecting magnetic resonance signals with a specific frequency suppressed and generating an image based on the magnetic resonance signal with the specific frequency suppressed, such as a SPSP (spectral spatial) method Is known (for example, see Non-Patent Documents 1 and 2).
In the SPSP method, a predetermined series of RF waves is applied to a subject together with a gradient magnetic field that vibrates positively and negatively. As a result, a magnetic resonance signal in which the frequency of a desired tissue such as fat is suppressed can be obtained from a predetermined region of the subject site of the subject.
The accuracy when selecting a desired region of the region to be examined is called spatial selectivity. In addition, suppressing the frequency of fat in a magnetic resonance signal is called fat suppression, and obtaining a magnetic resonance signal in a specific frequency band for fat suppression or the like is called frequency selectivity.
Fritz Schick et al., “Highly Selective Water and Fat Imaging Applying Multislice Sequences without Sensitivity to B1 Field Inhomogeneities ”, Magnetic Resonance in Medicine 1997, 38: p269-274 J Forster et al., “Slice-Selective Fat Saturation in MR Angiography Using Spatial-Spectral Selective Prepulses”, Journal・ Of Magnetic Resonance Imaging 1998, 8 (3): p583-589

  By the way, the time during which an RF wave that determines spatial selectivity in the SPSP method can be applied to the subject is determined by the magnitude of the static magnetic field. For this reason, for example, when the size of the static magnetic field is limited due to the performance of hardware, the time during which the RF wave can be applied is limited. As a result, inconvenience that sufficient space selectivity cannot be obtained may occur.

  Further, the SPSP method has a disadvantage that a sufficient fat suppression effect cannot be obtained due to the influence of residual magnetism generated due to a gradient magnetic field that vibrates positively and negatively applied to a subject.

Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of relaxing restrictions on spatial selectivity and frequency selectivity when magnetic resonance signals are generated spatially and frequency-selectively. is there.
Another object of the present invention is to provide a magnetic resonance image generation method capable of relaxing restrictions on spatial selectivity and frequency selectivity when magnetic resonance signals are generated spatially and frequency-selectively. There is also to do.

  A magnetic resonance imaging apparatus according to the present invention includes an RF wave applying unit that applies an RF wave to a test site of a subject in a static magnetic field, and a gradient magnetic field that sets position information by giving position information to the test site And a detection means for detecting a magnetic resonance signal from protons in the selected region, and generating image data of the region to be examined based on the magnetic resonance signal detected by the detection means A magnetic resonance imaging apparatus that combines the RF wave application means, the gradient magnetic field application means, and the detection means, and a first pulse sequence for suppressing or exciting target protons in the selected region; A control method for executing the second pulse sequence for collecting the magnetic resonance signal in which the frequency component of the resonance frequency of the target proton is suppressed or excited from the region including the selected region. In the first pulse sequence, the control means makes the magnitude of positive and negative polarities of the pulses of the gradient magnetic field having the same area and different polarities asymmetric, and the gradient having a smaller polarity. The RF wave is applied together with the magnetic field pulse.

  Further, the magnetic resonance image generation method according to the present invention includes an RF wave applying means for applying an RF wave to a test site of a subject in a static magnetic field, and setting a selection region by giving position information to the test site. A magnetic resonance image generation method using a magnetic resonance imaging apparatus that generates image data of the region to be examined based on a magnetic resonance signal from protons in the selected region. The RF wave applying means and the gradient magnetic field applying means execute a pulse sequence for suppressing or exciting the target proton in the selected region, and the frequency component of the resonance frequency of the target proton is suppressed or excited. And a magnetic resonance signal generation step for generating the magnetic resonance signal from a region including the selected region. In the magnetic resonance signal generation step, the area is the same and the polarity is The pulse of the gradient magnetic field whose polarity is not intended is applied by the gradient magnetic field applying unit, and the RF wave applying unit together with the pulse of the gradient magnetic field having a smaller polarity is applied by the RF wave applying unit. This is a magnetic resonance image generation method in which a wave is applied.

In the present invention, the control means combines the RF wave applying means and the gradient magnetic field applying means and drives them according to a predetermined pulse sequence to suppress or excite target protons in a predetermined selected region of the test site. In this pulse sequence, the control means applies a gradient magnetic field pulse having the same area but different polarity and having a polarity of this polarity that is not targeted by the gradient magnetic field application means. Along with the pulse, an RF wave is applied by an RF wave applying means. Thereby, a magnetic resonance signal in which the frequency component of the resonance frequency of the target proton is suppressed or excited in the selected region is obtained from the test site.
This magnetic resonance signal from the region to be examined is detected by the detection means.

  According to the present invention, when magnetic resonance signals are generated in a space-selective and frequency-selective manner, it is possible to relax restrictions on space selectivity and frequency selectivity.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

[First Embodiment]
First, a configuration example of an MR (Magnetic Resonance) imaging apparatus according to the first embodiment of the present invention will be described.
FIG. 1 is a schematic configuration diagram schematically showing the configuration of the MR imaging apparatus 100 according to the first embodiment.
The MR imaging apparatus 100 has a main body part 110 and a console part 280. In FIG. 1, the main body 110 is depicted as a schematic perspective perspective view of the main part.
The main body 110 further includes a magnet system and a driving unit 250.

The magnet system includes a pair of magnet portions 150a and 150b for generating a static magnetic field, gradient coil portions 160a and 160b, and RF (Radio Frequency), one of which is disposed to face each other in the casing 141 of the main body 110. ) Includes coil portions 180a and 180b.
For example, the RF coil portions 180a and 180b, the gradient coil portions 160a and 160b, and the static magnetic field generating magnet portions 150a and 150b are arranged to face each other from the inside. A bore 141a in which a subject (not shown) is arranged is formed between the innermost RF coil portions 180a and 180b.
An MR imaging apparatus 100 as shown in FIG. 1 is called an open type MR imaging apparatus because the casing 141 is configured in a shape that allows most of the bore 141a to be released.

The drive unit 250 includes an RF coil drive unit 12, a gradient coil drive unit 13, a data collection unit 14, and a magnet system control unit 15. Although these are drawn apart from the main body 110 in order to clearly show the connection relationship in FIG. 1, they are actually provided in the housing 141 of the main body 110, for example.
A magnet system control unit 15 is connected to the RF coil drive unit 12, the gradient coil drive unit 13, and the data collection unit 14, respectively.
Further, the RF coil driving unit 12 and the data collecting unit 14 are connected to the RF coil units 180a and 180b. The gradient coil drive unit 13 is connected to the gradient coil units 160a and 160b.

  One embodiment of the RF wave applying means in the present invention is configured to include the RF coil portions 180 a and 180 b and the RF coil driving portion 12. One embodiment of the gradient magnetic field applying means in the present invention includes the gradient coil units 160a and 160b and the gradient coil drive unit 13. Further, one embodiment of the detecting means is configured to include the RF coil units 180a and 180b and the data collecting unit 14. One embodiment of the control means in the present invention corresponds to the magnet system control unit 15.

The static magnetic field generating magnets 150a and 150b are configured using, for example, permanent magnets. A static magnetic field is formed in the bore 141a by the static magnetic field generating magnets 150a and 150b arranged to face each other.
The direction of the static magnetic field formed by the static magnetic field generating magnet units 150a and 150b is, for example, the y direction. As shown in FIG. 1, in the present embodiment, since the static magnetic field generating magnet portions 150a and 150b are arranged to face each other in the vertical direction, the vertical direction is the y direction. A longitudinal static magnetic field is also called a vertical magnetic field.
In addition, two directions orthogonal to the y direction are defined as an x direction and a z direction, respectively, as shown in FIG. Although not shown, the subject is often positioned in the bore 141a so that the body axis direction from the head to the leg of the subject coincides with the z direction.

  In the open type MR imaging apparatus, the magnetic field strength of the static magnetic field is currently about 0.2 to 0.7 Tesla (T). A magnet system of about 0.2 to 0.7 Tesla is called a low and medium magnetic field system.

The gradient coil sections 160a and 160b have three systems, that is, three pairs of gradient coils, in order to give the magnetic resonance signals detected by the RF coil sections 180a and 180b three-dimensional position information. Using these gradient coils, the gradient coil sections 160a and 160b generate gradient magnetic fields that give gradients in three directions of x, y, and z to the strength of the static magnetic field formed by the static magnetic field generating magnet sections 150a and 150b. .
Of these three-direction gradient magnetic fields, one is a slice selection gradient magnetic field for selecting a slice at the test site, one is a phase encoding gradient magnetic field, and the other is a reading gradient magnetic field (also called a frequency encoding gradient magnetic field). .

The RF coil units 180a and 180b include a transmission RF coil and a reception RF coil. The RF coil for transmission applies an RF band magnetic field for tilting the spin rotation axis of the proton at the test site to the test site of the test subject in the static magnetic field. Hereinafter, the RF magnetic field is simply referred to as an RF wave.
When the application of the RF wave by the transmitting RF coil is stopped, a magnetic resonance signal having a frequency component having the same resonance frequency as the frequency band of the applied RF wave is generated from the test site due to the spin of the test site. Re-radiated. The receiving RF coil detects this magnetic resonance signal from the site to be examined.

The RF coil for transmission and the RF coil for reception may be shared by the same coil. For example, the RF coil of the RF coil unit 180a is used as a transmission coil, and the RF coil of the RF coil unit 180b is used as a reception RF coil. As such, a dedicated coil may be used.
In addition to the RF coil housed in the housing 141 like the RF coil portions 180a and 180b, a dedicated RF coil corresponding to the test site such as the head, abdomen, or shoulder of the subject is used for transmission / reception RF. It can also be used as a coil.
Note that the range of the frequency of the RF wave is, for example, a range of 2.13 MHz to 85 MHz.

The gradient coil drive unit 13 transmits a gradient magnetic field excitation signal for generating a gradient magnetic field that gives a three-dimensional gradient to the strength of the static magnetic field to the above-described three systems of gradient coils.
In response to the gradient magnetic field excitation signal from the gradient coil drive unit 13, the gradient coil units 160a and 160b are driven to generate a three-dimensional gradient in the strength of the static magnetic field, thereby defining the imaging target region in the subject. it can. An imaging region is defined in slice units of a predetermined thickness. FIG. 1 shows an example in which a plurality of slices S parallel to the xy plane are arranged in the z direction. However, the arrangement shown in FIG. 1 is merely an example, and slices can be set at arbitrary positions in the bore 141a.

  The RF coil driving unit 12 applies an RF wave to the subject in the bore 141a by applying an RF wave excitation signal to the RF coil units 180a and 180b. By applying this RF wave, the inclination of the spin rotation axis of the proton at the test site can be changed.

The data collection unit 14 takes in the magnetic resonance signals detected by the RF coil units 180a and 180b and collects them as original data for magnetic resonance image generation.
For example, after collecting all data for generating one image, the data collection unit 14 transmits the collected data to the data processing unit 18 of the console unit 280 described later.
Further, the data collection unit 14 also transmits a part of the data related to the acquired magnetic resonance signal to the magnet system control unit 15.

The magnet system control unit 15 receives the command signal from the MR imaging device control unit 17 of the console unit 280, and the RF coil driving unit 12, the gradient coil so that the RF wave, the gradient magnetic field, and the magnetic resonance signal follow a predetermined pulse sequence. The drive unit 13 and the data collection unit 14 are controlled.
A pulse sequence is a pulse waveform of an RF wave, a gradient magnetic field, and a magnetic resonance signal (hereinafter simply referred to as a pulse) along the elapsed time. Each pulse has a shape defined by the pulse sequence. The RF wave excitation signal and the gradient magnetic field excitation signal are input from the RF coil driving unit 12 and the gradient coil driving unit 13 to the RF coil units 180a and 180b and the gradient coil units 160a and 160b, respectively.

The console unit 280 is for performing various operations such as input of command parameters to the magnet system control unit 15 and input of an imaging start command in order to obtain a magnetic resonance image of the subject by the main body unit 110. .
As shown in FIG. 1, the console unit 280 includes an MR imaging device control unit 17, a data processing unit 18, an operation unit 19, and a display unit 20.

The MR imaging device control unit 17 is connected to the data processing unit 18 and the display unit 20. The data processing unit 18 is connected to the display unit 20.
Further, a data collection unit 14 is connected to the data processing unit 18, and an operation unit 19 is connected to the MR apparatus control unit 17.

  The operation unit 19 is realized by an input device such as a keyboard and a mouse, for example. A command signal from an operator who operates the console unit 280 is input to the MR apparatus control unit 17 via the operation unit 19.

The MR imaging device control unit 17 is realized by, for example, hardware for calculation such as a CPU and software such as a program for driving the hardware.
The above program is stored in a storage unit (not shown) realized by, for example, a RAM (Random Access Memory) or a hard disk drive.
The MR imaging device control unit 17 controls the magnet system control unit 15, the data processing unit 18, and the display unit 20 in an integrated manner so as to realize an instruction from the operator input via the operation unit 19. If there is a restriction such as a hardware restriction of the main body 110, the MR apparatus control unit 17 displays on the display unit 20 that the input command cannot be executed.

  The data processing unit 18 performs predetermined processing such as arithmetic processing and image processing on the magnetic resonance signal data transmitted from the data collection unit 14 based on instructions from the operator via the operation unit 19 and the MR device control unit 17. The process which produces | generates a magnetic resonance image is performed. The image generated by the data processing unit 18 can be stored in a storage unit (not shown).

The image generated by the data processing unit 18 is appropriately displayed on the display unit 20 in response to a request from the operator.
The display unit 20 is realized by a monitor such as a liquid crystal display panel or a CRT (Cathode-Ray Tube), for example.
The display unit 20 also displays an operation screen for operating the MR imaging apparatus 100.

With the above configuration, a magnetic resonance image of the subject can be generated using the MR imaging apparatus 100. Hereinafter, an example of a pulse sequence for generating a magnetic resonance signal used for generating a magnetic resonance image will be described with reference to FIG.
In the pulse sequence shown in FIG. 2, the horizontal axis represents an elapsed time t that proceeds from the left side to the right side. Each graph includes an RF wave application pulse sequence RF, a slice selection gradient magnetic field application pulse sequence G_slice, a phase encoding gradient magnetic field application pulse sequence G_phase, a read gradient magnetic field application pulse sequence G_read, and a magnetic resonance signal generation sequence Signal in order from the top of FIG. Each represents.

The sequence RF indicates the waveform of the RF wave applied to the subject from the RF coil units 180a and 180b.
The sequence G_slice represents a waveform of a slice selection gradient magnetic field pulse applied to the test site by the gradient coil units 160a and 160b for selecting an imaging slice of the test site.
The sequence G_phase represents a waveform of a phase encoding gradient magnetic field pulse applied to the test site by the gradient coil sections 160a and 160b for use in encoding position information in the phase direction of the test subject.
The sequence G_read represents a waveform of a read gradient magnetic field pulse applied to the test site by the gradient coil units 160a and 160b in order to emit a magnetic resonance signal from the test site to which the RF wave is applied by the RF coil units 180a and 180b. ing.
The sequence Signal represents the magnetic resonance signal 54 that is emitted from the test site and detected by the RF coil units 180a and 180b.

  The step of applying the RF wave and performing phase encoding with the phase encoding gradient magnetic field is repeated a predetermined number of times while changing the magnitude of the phase encoding gradient magnetic field in accordance with the pixel size of the target image. This operation is expressed by a plurality of phase encoding gradient magnetic field pulses 52 in the sequence G_phase of FIG.

  As shown in FIG. 2, the pulse sequence for generating magnetic resonance signals according to the first embodiment is roughly divided into a first pulse sequence PS1 and a second pulse sequence PS2.

The first pulse sequence PS1 is a pulse sequence that can suppress or excite a predetermined frequency only in a predetermined slice among a plurality of slices S set in a subject in a static magnetic field.
As a pulse sequence that can generate magnetic resonance signals spatially and selectively in this manner, for example, a SPSP (spectral spatial) method pulse sequence can be used.
Since the SPSP method is described in, for example, Non-Patent Documents 1 and 2 described above, detailed description is omitted. However, as shown in the RF wave 50a and the slice selective gradient magnetic field pulse 51a in FIG. By applying an RF wave having a predetermined waveform while continuously applying a slice selective gradient magnetic field pulse whose polarity changes to, a region for suppressing or exciting the target proton can be selected.

  In the following, a case where fat is the target proton and the frequency component in the resonance frequency band of fat is suppressed in the magnetic resonance signal 54 by the first pulse sequence PS1 will be described as an example. However, in the SPSP method, it is possible to excite the frequency components in the resonance frequency band of fat by appropriately changing the waveforms of the RF wave 50a and the slice selective gradient magnetic field pulse 51a. Since the SPSP pulse sequence is a frequency-selective pulse sequence, it is not limited to fat, and for example, it is possible to suppress or excite frequency components in the resonance frequency band of water.

In a method using a spatially selective and frequency selective pulse sequence such as the SPSP method, one cycle time TW in which an RF wave 50a for determining spatial selectivity can be applied is almost equal to the magnitude of the static magnetic field. Determined. For example, when the magnitude of the static magnetic field is 0.35 Tesla (T), the cycle time TW is about 6 to 8 ms.
In order to improve the spatial selectivity by reducing the thickness of the slice S, it is necessary to apply a larger slice selective gradient magnetic field pulse 51a. However, the rise time and fall time of the slice selective gradient magnetic field pulse 51 a, that is, the slope DK has a limit depending on the hardware performance of the MR imaging apparatus 100. For this reason, when attempting to increase the slice selective gradient magnetic field pulse 51a, the length RW1 of the flat portion to which the actual pulse of the RF wave 50a can be applied becomes shorter in each pulse of the slice selective gradient magnetic field pulse 51a.

In the present embodiment, in order to make the length WF as long as possible, the slice selection gradient magnetic field pulse 51a applied in the first pulse sequence has the same area and the magnitude of the polarity in the positive and negative polarities. Pulse.
That is, the areas of the positive pulse PLP and the negative pulse PLN of the slice selective gradient magnetic field pulse 51a are equal. In the present embodiment, the area of each pulse PLP, PLN is ar1. Further, the polarity magnitude HT1 of the positive pulse PLP is different from the polarity magnitude HT2 of the negative pulse PLN, and the magnitude is asymmetric with respect to the axis of zero.

In the present embodiment, for example, the magnitude HT2 of the negative pulse PLN is made larger than the magnitude HT1 of the positive pulse PNP. Thus, when the slope DK is constant, the length of the flat portion in the positive pulse PLP is longer than that in the case of the negative pulse PLN.
In the present embodiment, in order to apply the RF wave 50a as long as possible, each pulse of the RF wave 50a is applied together with a positive pulse PLP having a smaller polarity and hence a longer flat portion. .

Further, it is known that when a permanent magnet is used for the static magnetic field generating magnets 150a and 150b, there is a hysteresis that generates residual magnetism. Since the magnetic field strength of the static magnetic field changes due to the residual magnetism, the magnetic resonance signal is affected, and a sufficient fat suppression effect cannot be obtained. The magnetic resonance signal 54 obtained in the second pulse sequence PS2 is a desired signal. May be adversely affected.
In the present embodiment, since the magnitude HT1 of the positive pulse PLP and the magnitude HT2 of the negative pulse PLN are different, the influence of residual magnetism can be suppressed. The details are shown below.

FIG. 4 is a diagram for describing hysteresis of residual magnetism. The horizontal axis in FIG. 4 represents the magnitude of the slice selective gradient magnetic field pulse 51a, that is, the magnitude of the gradient, and the vertical axis represents the residual magnetism.
As shown in FIG. 4, the residual magnetism generated in the static magnetic field generating magnets 150a and 150b using a permanent magnet has a hysteresis whose magnitude changes depending on the path of change in the magnitude of the gradient. It has been known. Consider a loop in which the change in gradient magnitude is from g to -g as shown in FIG. At this time, for example, it is assumed that the gradient magnitude is changed from the point where the gradient magnitude is −g / 2 to the direction of the arrow in the figure. At the point where the magnitude of the gradient is -g / 2, the residual magnetism is zero.
Even if the magnitude of the gradient is increased in the negative direction to -g according to the loop and then returned to -g / 2 again, the remanence is not zero and -M remains. In order to eliminate this residual magnetism, it is necessary to increase the magnitude of the gradient to g / 2.
In other words, in a loop in which the magnitude of the gradient is continuously changed from g to −g, the magnitude of the gradient is −g to g / It is necessary to change continuously up to 2 or from g to -g / 2.
As described above, it is known that the residual magnetism is eliminated when a gradient magnetic field having the opposite polarity and the absolute value of 2: 1 is continuously applied. This property is applicable to almost all magnet systems, regardless of the magnitude of the static magnetic field to be formed, if the magnet system uses permanent magnets.

Therefore, in this embodiment, in order to suppress the influence of the residual magnetism, the ratio of the positive pulse magnitude HT1 to the negative pulse magnitude HT2 is HT1: HT2 = 1: 2 as shown in FIG. And
As a result, if the residual magnetism at the start of the first pulse sequence PS1 is 0, the residual will be first at the time t1 shown in FIG. 2 when considering each pulse application cycle of the RF wave 50a. The influence of magnetism can be eliminated.
It should be noted that not only when HT1: HT2 = 1: 2, but if the magnitude HT1 and the magnitude HT2 are different, the influence of residual magnetism can be suppressed to some extent.

In the second pulse sequence PS2 after the execution of the first pulse sequence PS1, a pulse sequence such as a spin echo method, a gradient echo method, an echo planar imaging method or the like is appropriately applied. can do.
FIG. 2 shows, as an example, a pulse sequence for obtaining a magnetic resonance signal from a subject by a gradient echo method.

  In the gradient echo method, as shown in FIG. 2, an RF wave 50b is applied to a subject in a state where a slice is selected by applying a slice selection gradient magnetic field pulse 51b. The slice selected at this time is a slice targeted for fat suppression in the first pulse sequence PS1.

  After applying the RF wave 50b for generating the magnetic resonance signal 54, the read gradient magnetic field is applied while applying the phase encode gradient magnetic field pulse 52 and applying the position information in the phase encode direction as shown in FIG. A pulse 53 is applied to the test site. By applying the readout gradient magnetic field pulse 53, the magnetic resonance signal 54 as an echo from the slice selected by the slice selection gradient magnetic field pulse 51b is detected by the RF coil units 180a and 180b.

The time from the center of the RF wave 50b applied to acquire the magnetic resonance signal 54 to the center of the magnetic resonance signal 54 is referred to as an echo time TE.
The time from the start of the first pulse sequence PS1 to the end of the second pulse sequence PS2 is referred to as a repetition time TR.

As described above, in the present embodiment, the slice selective gradient magnetic field pulse 51a is applied in order to suppress or excite the resonance frequency of protons in a specific tissue (for example, fat) in a specific slice at a test site. In the present embodiment, for the slice selective gradient magnetic field pulse 51a having the same positive and negative area, the magnitude HT1 of the positive pulse PLP is made smaller than the magnitude HT2 of the negative pulse PLN, and the axis is asymmetric with respect to the zero axis. I have to. Thereby, in the positive pulse PLP having a small polarity, the length RW1 of the flat portion is larger than that of the negative pulse PLN. Therefore, even if the cycle time TW in which the RF wave 50a can be applied is limited due to hardware limitations or the like, if each pulse of the RF wave 50a is applied together with the positive pulse PLP, the RF wave 50a is longer. Can be applied. Since the application time of the RF wave 50a is directly related to the spatial selectivity by the first pulse sequence PS1, according to the present embodiment, a desired slice can be selected more accurately.
In the present embodiment, the influence of residual magnetism can be eliminated by setting HT1: HT2 = 1: 2. As a result, a change in the magnetic field strength of the static magnetic field due to the residual magnetism can be prevented, a more reliable frequency selection effect can be obtained, and the image quality of the magnetic resonance image can be improved.
Furthermore, even if the magnitudes HT1 and HT2 of the positive and negative pulses are not necessarily 1: 2, the influence of residual magnetism can be suppressed to some extent, and the length of the flat portion RW1 is changed according to the magnitudes HT1 and HT2. Therefore, the waveform of the pulse sequence can be changed as appropriate, and the degree of freedom in designing the pulse sequence is improved.

The cycle time TW in which the RF wave 50a can be applied is directly related to the magnitude of the static magnetic field, and the cycle time TW decreases as the magnetic field strength of the static magnetic field decreases. Since the application time of the pulse of the RF wave 50a can be extended within a limited cycle time TW, this embodiment can be said to be particularly effective in a low / medium magnetic field magnet system.
Since a permanent magnet is mainly used for a low-medium magnetic field magnet system, this embodiment can be said to be particularly effective for a permanent magnet system.

[Second Embodiment]
By changing the waveform of the slice selective gradient magnetic field pulse 51a, the influence of residual magnetism can be eliminated at an earlier time. A slice selective gradient magnetic field pulse for that purpose will be described below.
FIG. 3 is a diagram showing a pulse sequence PS3 used instead of the first pulse sequence PS1 in the second embodiment.
Since the second embodiment is the same as the first embodiment except that the pulse sequence PS3 is used instead of the first pulse sequence PS1, detailed description of the same parts is omitted.

Prior to the first pulse sequence PS1 according to the first embodiment, the pulse sequence PS3 has a slice selection gradient magnetic field having the same area and a different polarity as the first pulse of the slice selection gradient magnetic field pulse 51a in the first pulse sequence PS1. The pulse PLR is added.
However, in FIG. 3, only the graph of the sequence RF and the sequence G_slice and the axis of time t are shown. Similar to the first embodiment, the time t flows from the left side to the right side.

For example, in the present embodiment, a negative pulse PLR is provided before the positive slice selective gradient magnetic field pulse PLP so as to be linked to the pulse PLP. The area of this pulse PLR is the same as the area ar1 of each pulse of the slice selective gradient magnetic field pulse 51a.
If the ratio of the magnitude HT3 of the negative pulse PLR to the magnitude HT1 of the positive pulse PLP is HT1: HT3 = 1: 2 as in the first embodiment, at the time t2 shown in FIG. First, the influence of residual magnetism can be eliminated.

Comparing the first pulse sequence PS1 and the pulse sequence PS3 according to the second embodiment, in the first pulse sequence PS1, the time when the application of the two pulses 50a1 and 50a2 of the RF wave 50a is completed is time t1. On the other hand, in the pulse sequence PS3, the time when only the pulse 50a1 ends is the time t2 at which the influence of residual magnetism can be eliminated.
Thus, according to the second embodiment, in addition to the same effect as that of the first embodiment, the effect that the influence of residual magnetism can be eliminated earlier can be obtained.

In addition, this invention is not limited to the above-mentioned embodiment, It can change suitably in a claim.
For example, the present invention can be applied not only to the MR imaging apparatus 100 having an open type magnet system as shown in FIG. 1, but also to an MR imaging apparatus having a so-called cylindrical type magnet system in which a bore is formed in a cylindrical shape. It is. For forming a static magnetic field, not only a permanent magnet but also a normal conducting magnet or a superconducting magnet may be used.
In the above embodiment, the case where a gradient echo method sequence is used as the second pulse sequence PS2 has been described. However, in the present invention, other magnetic resonance signal acquisition sequences such as a spin echo method are used. Also good. As the first pulse sequence PS1, a pulse sequence other than the SPSP method may be applied as long as magnetic resonance signals can be generated in a space-selective and frequency-selective manner.

  The present invention can be suitably used in the field of magnetic resonance imaging in which a subject is imaged using a magnetic resonance signal.

1 is a schematic configuration diagram schematically showing a configuration of an MR imaging apparatus according to a first embodiment of the present invention. It is a figure which shows an example of the pulse sequence used for magnetic resonance signal generation | occurrence | production in 1st Embodiment of this invention. It is a figure which shows an example of the principal part of the pulse sequence used for magnetic resonance signal generation | occurrence | production in 2nd Embodiment of this invention. It is a figure which shows the hysteresis of a residual magnetism.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 ... RF coil drive part 13 ... Gradient coil drive part 14 ... Data collection part 15 ... Magnet system control part (control means)
50a, 50b ... RF wave 51a, 51b ... Slice selection gradient magnetic field pulse 52 ... Phase encoding gradient magnetic field pulse 53 ... Reading gradient magnetic field pulse 54 ... Magnetic resonance signal 100 ... MR imaging device 110 ... Main body 150a, 150b ... For generating static magnetic field Magnet part 160a, 160b ... Gradient coil part 180a, 180b ... RF coil part 250 ... Drive part
PS1 ... 1st pulse sequence
PS2 ... Second pulse sequence
PLP ... positive pulse
PLN ... Negative pulse

Claims (16)

RF wave applying means for applying an RF wave to a test site of a subject in a static magnetic field, gradient magnetic field applying means for applying a gradient magnetic field for setting a selection region by giving position information to the test site, and A magnetic resonance imaging apparatus comprising: detection means for detecting a magnetic resonance signal from protons in a selected region; and generating image data of the region to be examined based on the magnetic resonance signal detected by the detection means,
In combination with the RF wave application means, the gradient magnetic field application means and the detection means,
The first pulse sequence for suppressing or exciting the target proton in the selected region and the magnetic resonance signal in which the frequency component of the resonance frequency of the target proton is suppressed or excited are collected from the region including the selected region. Control means for executing the second pulse sequence,
The control means includes the first pulse sequence,
Making the magnitudes of the positive and negative polarities of the pulses of the gradient magnetic field different in polarity with the same area, asymmetric,
A magnetic resonance imaging apparatus that applies the RF wave together with the pulse of the gradient magnetic field having a smaller polarity. 2. The magnetic resonance imaging apparatus according to claim 1, wherein each magnitude of the asymmetric polarity of the gradient magnetic field pulse is such a magnitude as to cancel out an influence of residual magnetism due to the gradient magnetic field pulse. The magnetic resonance imaging apparatus according to claim 2, wherein a ratio of the polarities of the gradient magnetic field pulses is 1: 2. The magnetic resonance imaging apparatus according to claim 3, wherein the static magnetic field is formed by a permanent magnet. The control means includes
The pulse of the gradient magnetic field having the same area and different polarity as the first pulse of the gradient magnetic field in the first pulse sequence is applied before the first pulse sequence. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field is a low-medium magnetic field having a magnetic field strength of 0.2 to 0.7 Tesla. The magnetic resonance imaging apparatus according to claim 1, wherein the first pulse sequence is a pulse sequence for suppressing or exciting a resonance frequency of fat protons. The magnetic resonance imaging apparatus according to claim 1, wherein the first pulse sequence is a pulse sequence for suppressing or exciting a resonance frequency of protons in water. An RF wave applying means for applying an RF wave to a test site of a subject in a static magnetic field, and a gradient magnetic field applying means for applying a gradient magnetic field for assigning position information to the test site and setting a selection region are provided. , A magnetic resonance image generation method using a magnetic resonance imaging apparatus for generating image data of the test site based on a magnetic resonance signal from protons in the selected region,
The magnetic field in which the frequency component of the resonance frequency of the target proton is suppressed or excited by executing a pulse sequence for suppressing or exciting the target proton in the selected region by the RF wave applying means and the gradient magnetic field applying means. A magnetic resonance signal generating step of generating a resonance signal from a region including the selected region;
In the magnetic resonance signal generating step, the gradient magnetic field pulse having the same area but different polarity and having the polarity of the polarity not targeted is applied by the gradient magnetic field applying unit, and the polarity is smaller. A magnetic resonance image generating method in which the RF wave is applied by the RF wave applying means together with a pulse of a gradient magnetic field. 10. The magnetic resonance image generation method according to claim 9, wherein the magnitudes of the asymmetric polarities of the gradient magnetic field pulses are such that the influence of residual magnetism due to the gradient magnetic field pulses is canceled out. The magnetic resonance image generation method according to claim 10, wherein the ratio of the magnitudes of the polarities of the gradient magnetic field pulses is 1: 2. The magnetic resonance image generation method according to claim 11, wherein the static magnetic field is formed by a permanent magnet. The magnetic resonance image generation method according to claim 9, wherein a pulse of a gradient magnetic field having the same area and a different polarity as the first pulse of the gradient magnetic field in the pulse sequence is applied before the pulse sequence. The magnetic resonance image generation method according to claim 9, wherein the static magnetic field is a low-medium magnetic field having a magnetic field strength of 0.2 to 0.7 Tesla. The magnetic resonance image generation method according to claim 9, wherein the pulse sequence is a pulse sequence for suppressing or exciting a resonance frequency of fat protons. The magnetic resonance image generation method according to claim 9, wherein the pulse sequence is a pulse sequence for suppressing or exciting a resonance frequency of protons in water.

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